This invention relates to microwave switching devices. More particularly, this invention relates to an inverted PIN diode switch that does not use PIN diodes or DC blocking capacitors as series elements in the signal path and that does not require one-quarter wavelength tuning elements to transform impedances within the switch.
For many years, switches have been used in the electrical arts to provide a means for isolating a portion of an electrical circuit. In its simplest form, a single pole/single throw switch resides in one of two positions. In a "closed" position, the switch allows a signal to pass from an input port to an output port. In an "open" position, the switch prevents a signal from passing from the input port to the output port. A theoretically perfect switch has no series impedance or shunt admittance in the "closed" position and has either infinite series impedance, infinite shunt admittance, or both in the "open" position.
The earliest switches had mechanical contacts and were manually switched from the "open" position to the "closed" position and back again. Since that time, mechanical switches have been improved to operate automatically and remotely and are still used for many applications. In the particular application to microwave signals in the 12 to 40 Gigahertz range, however, mechanical switches operate too slowly for most applications. Further, since all mechanical switches fail over time due to the breakage of the moving parts, mechanical switches are inappropriate where complete reliability is required.
In response to the shortcomings of mechanical switches, the solid-state switch was developed. Solid state switches have no moving parts and last indefinitely. The solid state switches use semiconductor devices in a variety of configurations to provide the "open" and "closed" positions and often employ PIN diodes as their controllable elements. PIN diodes, as is well known, are diodes that are formed from a silicon wafer containing nearly equal P type and N type impurities. P type impurities are diffused from one side and N type impurities are diffused from the other side in a manner common to all diodes. However, in PIN diodes, a barrier layer of undoped silicon is allowed to remain between the doped regions. The barrier layer causes the PIN diodes to be slow in their action compared to regular diodes. Higher frequency applied signals, such as those in the radio frequency ("CRF") and microwave frequencies do not cause the PIN diodes to become forward biased during the positive portion of the signal cycle. In their unbiased states, the PIN diodes have a series resistance of approximately 10 kilo ohms and a small junction capacitance. In their biased state, the PIN diodes have a series resistance of approximately one to two ohms. Resultantly, because of their switching and electrical characteristics, PIN diodes function well in high frequency solid state switch applications.
One particular switch configuration designed to operate in the RF and microwave frequency spectrums uses PIN diodes as series element within the switch. In this configuration, the switch is "closed" when the PIN diodes are forward biased and "open" when the PIN diodes are unbiased. Particular examples of this type of structure are described in U.S. Pat. Nos. 4,525,863 to Stites and 3,475,700 to Ertel. The use of PIN diodes as series elements in a switch, however, causes significant problems. First, forward biasing the PIN diodes introduces DC voltages and transient switching voltages on the signal path. The DC voltages and transient switching voltages feed into the external circuits if not blocked and can damage the external circuits. Second, the junction capacitance of the PIN diodes allows a significant portion of the coupled microwave signal to pass through the switch when the switch is in the "open" position. Therefore, these switches also may provide inferior isolation when the switch is in the "open" position.
Another particular switch configuration that uses PIN diodes, and that is designed to operate in the RF and microwave spectrum, employs PIN diodes with their anodes connected to a transmission line segment that spans from the switch input to the switch output. The cathodes of the PIN diodes are grounded. In the "closed" position, the PIN diodes are unbiased and the coupled signals are allowed to pass from the switch input to the switch output. In the "open" position, the transmission line is biased at a positive DC voltage sufficient to forward bias the PIN diodes therefore causing the PIN diodes to provide a shunt to ground. A significant shortcoming of this switch configuration is that the DC bias voltage is coupled to the transmission line segment. Therefore, DC blocking capacitors must be used as series elements at each end of the transmission line segment to block the DC voltages from entering the coupled circuitry. While the DC blocking capacitors provide satisfactory isolation, they introduce a series impedance in the signal path that attenuates the coupled signal. Typical attenuation caused by the capacitors when the switch is in the "closed" position is between 3 to 3.5 decibels. Because of the high cost of signal amplification in the RF and microwave spectrums, such attenuation is unsatisfactory.
Another particular switch configuration using PIN diodes employs one-quarter wavelength transmission line segments to transform impedances from one portion of the switch to another portion of the switch. The circuit described in U.S. Pat. No. 5,193,218 to Shimo, for example, uses four one-quarter wavelength transmission line segments to provide either very large or very small apparent impedances at junction points on the signal path. When a very large apparent impedance is provided at a point where a quarter wavelength tuning branch joins the main signal path, the signal passes the quarter wavelength branch. However, when the apparent impedance of the quarter wavelength branch is small at the junction point, the signal is shunted along the quarter wavelength branch and does not pass along the signal path.
Switches employing quarter wavelength circuits have significant drawbacks, however. In standard microwave applications, the length of the quarter wavelength segments is large compared to the remainder of the circuit. Therefore, the size of the housing and the length of conductors for the switch is increased compared to other switches. Further, because the wavelength of a signal is inversely proportional to its frequency and because the length of the quarter wavelength lines is fixed, the switch is designed to operate at a single frequency. Therefore, the effectiveness of the quarter wavelength lines, and resultantly the switch, decreases substantially as the frequency of operation varies from the design frequency.